Embodiments according to the invention relate to a method for structuring a device layer of a substrate.
Several embodiments of the invention relate to a method for three-dimensional structuring of micromechanical structures.
Micromechanical devices, such as micro-optical devices, such as scanners, can be produced with volume micromechanical methods, for example on BSOI (bonded-silicon-on-insulator), wherein the devices are geometrically defined by “etching through” the device layer or device level, respectively, to a separation layer or intermediate layer, respectively, for example a BOX (buried oxide). The space below the actuators, where the carrier wafer or the carrier layer, respectively, is, is normally exposed for allowing unlimited vibration. In an “in-plane” arrangement of electrodes, for example, lateral electrodes can be used for exciting, for example, resonant micromechanical devices.
DE 199 63 382 A1 describes an example of a micromirror. A micromirror, in particular a micro vibration mirror with at least one largely cantilevered mirror surface is suggested, which is deflectable from the resting position by at least one torsion spring. Here, the mirror surface is connected to at least one support body by torsion beams guided at least approximately parallel to each other. Further, it is suggested to deflect the micromirror from its resting position via electrostatic or magnetic interaction.
FIGS. 9(A)-(C) show an example of a method 900 for producing a BSOI wafer. Here, in a first step 910, a carrier layer 120, which consists, for example, of silicon and has an oxide layer 130 at its surface, is connected to a device layer 110 having a thickness of 2 μm to 400 μm, which consists, for example, of silicon, which is shown in FIGS. 9(A) and 9(B). The oxide layer 130 on the surface of the carrier layer 120 serves as an intermediate layer 130 between the device layer 110 and the carrier layer 120. In this case, the intermediate layer 130 is also called BOX. In a second step 920, the device layer 110 is structured in its layer thickness and at its edges, which is shown in FIG. 9(C).
Further, FIGS. 10(A)-(D) show an example of a method 1000 for producing a micromechanical structure in or on a BSOI wafer, which is produced, for example, according to a method according to FIG. 9. In the method 1000, in a first step 1010, as shown in FIG. 10(A), isolating trenches are provided in the device layer 110. Therefore, it is necessitated to etch trenches into the silicon of the device layer 110, and to subsequently oxidize the walls of the trenches (as shown at reference number 1011) and to fill the trenches, for example with polysilicon 1012. This step is terminated by a planarizing CMP process (CMP: chemical-mechanical polishing) or etching back with plasma.
A second step 1020 as shown in FIG. 10(B) comprises generating and structuring a back-side mask 1022, generating and structuring conductive traces 1024 and bondpads 1025 (bondpads: pads for providing, for example, electric connections to other electronic devices), and generating and structuring, for example, a mirror 1026. Thereby the back-side mask is formed by a layer 1022 which is on the surface of the carrier layer 120 facing away from the device layer 110 and is removed at the positions where the carrier layer 120 is to be etched later.
By a third step 1030, as shown in FIG. 10(C), a front-side mask 1032 for structuring the micromechanical element is generated, the carrier layer 120 is removed at the positions 1021 defined by the back-side mask 1022, for example by TMAH (TMAH: tetramethylammonium hydroxide), and the intermediate layer 130 or the BOX is also removed at the positions 1023 where the carrier layer 120 has been removed.
A fourth step 1040, as shown in FIG. 10(D) comprises etching trenches 1042 into the device layer 110, wherein at the positions where trenches 1042 are generated, the device layer 110 is removed completely, and comprises subsequent removal of the front-side mask 1032. The trenches 1042 in the device layer 110 define the geometrical form of the micromechanical element.
Normally, the physical properties of these devices, such as natural frequency or mode splitting are defined by the geometry of the suspensions, such as length and width of the springs, in combination with the geometries of the vibration mass and can be adapted correspondingly, for example to the requirements of the respective application. However, for certain applications, the parametric space of these two dimensions is not sufficient for obtaining a satisfactory solution. Thus, there is basically the possibility of locally changing the thickness of the device layer. In other words, apart from the definition of the geometry in two dimensions, additionally, as a third dimension, the thickness of the device layer can be changed for adjusting the physical properties of the devices.
Several methods allow changing the thickness of the device layer by locally depositing and structuring thick layers, whereby the physical properties (e.g. the mass) can be changed. However, the long process times necessitated for depositing and etching are expensive. Additionally, large steps cause lithography problems.
Other methods generate stepping of an SOI layer (SOI: silicon-on-insulator) from the front, which causes a loss of planarity. This results, for example, in lithography problems, and resist spin-on is no longer easily possible. A two-stage etching process (etching the steps and exposing the device by means of etching, normally a plasma etching process) is expensive to control and implies double mask effort. If no mask residuals are to remain on the finished device layer of the device for avoiding layer tensions, the technology is difficult to realize with a double-resist mask.
Several methods generate stepping of the SOI layer from the backside, which also causes a loss of planarity with the same problems. When using a spray resist, only very coarse structures can be resolved and adjusting is imprecise. Additionally, focussing problems result when structuring in trenches. Also, handling the plates on the possibly previously processed and structured front side might cause damage of the already existing structures.